This invention relates generally to free-electron lasers and, more particularly, to free-electron lasers that use a resonator cavity to enhance and control the stimulated emission of radiation from an electron beam. In contrast to other laser types in which electrons may be bound to a single atom or molecule, or in which electrons may be free to move through the entire volume of a semiconductor, the free-electron laser produces stimulated emission from a beam of free electrons in a vacuum.
Basically, in a free-electron laser, a beam of relativistic electrons, that is, electrons that have been accelerated to speeds comparable with the speed of light, is passed through a transverse and periodic magnetic field, known as a "wiggler," which results in periodic transverse movement of the electrons. Light is emitted in the direction of the electron beam as a result of the interaction between the electrons and the magnetic field, and is fed back and forth through the wiggler by means of two opposed mirrors. Stimulated emission comes about through the interaction of the electromagnetic wave fed back and forth and the periodic magnetic structure.
U.S. Pat. No. 3,822,410 to Madey entitled "Stimulated Emission of Radiation in Periodically Deflected Electron Beam," describes a free-electron laser, an example of which is depicted in FIG. 1 accompanying this specification. A beam of electrons, indicated by dotted line 1, traveling at velocities approaching the speed of light, that is at relativistic velocities, passes through a series of alternating magnetic fields, which periodically deflect the paths of electrons transversely to their principal direction of movement. These magnetic fields and mechanisms for providing them, for maintaining the convergence of the electron beam, and for injecting and extracting the electron beam from the active region of the laser, are not depicted in detail in FIG. 1, but are simply represented by the lines 2 in the figure. Reference may be made to the aforementioned Madey patent for a more detailed description of this aspect of the free-electron laser. As pointed out in the Madey patent, either periodic electric or periodic magnetic fields may be used to deflect the electrons, although magnetic fields are more easily provided.
Within the active region of the free-electron laser (FEL) the beam of electrons 1 travels along and is deflected about a center line 3 of the FEL. Mirrors 4 and 5 operate together to form a resonator, the axis of which is coincident with the center line 3, and hence with the electron beam 1 within the active region of the FEL 2. As depicted in FIG. 1, mirrors 4 and 5 are spherical or parabolic in shape so as to focus radiation incident on them into a small cross-sectional area within the active region of the FEL 2. Typically, radiation is extracted from the device as a beam 6 passing through one of the mirrors, such as mirror 5, which is made only partially reflective.
As indicated in the Madey patent, a free-electron laser may be used to provide continuous lasing action at power levels as high as five megawatts. However, when one attempts to scale the operation of such free-electron lasers to even higher power levels, practical problems arise in that the mirrors used to form the resonator cannot withstand the high energy density of the incident radiation. For instance, for a free-electron laser having an active region of five meters in length, and resonator mirrors spaced seven meters apart, an energy density of 10.sup.8 watts/cm.sup.2 is incident on the mirrors and results in approximately 10.sup.4 watts being absorbed by the mirrors. This causes a significant heating problem for the mirrors.
One way to reduce the heating problem is to move the mirrors further apart. Because the radiation from the free-electron laser diverges slightly, the radiation spreads over a larger area of the mirrors as the spacing between the mirrors is increased. For instance, if the active region is approximately five meters long and the spacing between mirrors is increased from seven meters to approximately 200 meters, the energy density of the radiation incident on the mirrors is decreased by a factor of approximately 800. As a consequence of increasing the spacing between mirrors by a factor of 25 to 30, the power level can be increased by a factor of approximately 6,000 while maintaining the same energy density on the mirrors.
If the spacing between the mirrors 4 and 5 in FIG. 1 is increased significantly, it is apparent that, in order to maintain the center of the resonator formed by these mirrors coincident with the electron beam 1 within the active region of the FEL 2, the pointing accuracy of the mirrors must be improved in proportion to the increase in spacing between them. Unfortunately, at spacings of 10's to 100's of meters, the required pointing accuracies are extremely difficult to maintain. Accordingly, there is a need for a new technique for reducing the pointing accuracy requirement of cavity mirrors employed in free-electron lasers. The present invention satisfies this need, as will be apparent from the following summary.